The present disclosure relates to a fuel cell stack system such as a direct methanol fuel cell (DMFC) directly supplying methanol to a fuel electrode to cause a reaction, a fuel cell, an electrode used in the fuel cell, and an electronic device using them. Moreover, the present invention relates to a channel structure allowing a fluid (a liquid or a gas) to flow therethrough, a fuel cell such as a DMFC using the channel structure, and an electronic device including them. Further, the present invention relates to a channel structure suitable for a micro TAS (Total Analysis System), a fuel cell or the like.
Indicators of characteristics of a battery include energy density and output density. The energy density is an amount of energy storage of the battery per unit mass, and the output density is an output amount of the battery per unit mass. Lithium-ion secondary batteries combine two characteristics of relatively high energy density and remarkably high output density, and also have high perfection, so it is widely adopted as power sources for mobile devices. However, in recent years, the mobile devices tend to consume more power with performance enhancement, thereby further improvements in energy density and output density of the lithium-ion secondary batteries are desired.
Solutions to such an issue include changing an electrode material forming a cathode and an anode, improving a method of applying an electrode material, improving a method of sealing an electrode material, and the like, and research aimed at improving the energy density of the lithium-ion secondary batteries has been conducted. However, a hurdle to practical use is still high. Moreover, unless constituent materials used for the lithium-ion secondary batteries are changed, it is difficult to expect a drastic improvement in the energy density.
Therefore, the development of batteries with higher energy density as an alternative to the lithium-ion secondary batteries is urgently necessary, and fuel cells are considered as a promising candidate.
The fuel cell has a configuration in which an electrolyte is arranged between an anode (a fuel electrode) and a cathode (an oxygen electrode), and a fuel, and air or oxygen are supplied to the fuel electrode and the oxygen electrode, respectively. As a result, an oxidation-reduction reaction in which the fuel is oxidized by oxygen occurs in the fuel electrode and the oxygen electrode, and a part of chemical energy of the fuel is converted into electrical energy to be extracted.
Various types of fuel cells have been already proposed or prototyped, and some of them have been already put to practical use. These fuel cells are classified into types, that is, an alkaline fuel cell (AFC), a phosphoric acid fuel cell (PAFC), a molten carbonate fuel cell (MCFC), a solid electrolyte fuel cell (SOFC), a polymer electrolyte fuel cell (PEFC) and the like according to electrolytes used in the fuel cells. The PEFC is operable at a lower temperature than other types, for example, a temperature of approximately 30° C. to 130° C.
As the fuel for the fuel cells, various flammable substances such as hydrogen and methanol may be used. However, a gas fuel such as hydrogen is necessary to be stored in a storage cylinder, so the gas fuel is not suitable to size reduction. On the other hand, a liquid fuel such as methanol is advantageous in terms of easy storage. Especially, the DMFC does not need a reformer for extracting hydrogen from the fuel, so the DMFC has advantages that its configuration is simplified and its size is easily reduced.
In the DMFC, methanol as the fuel is typically supplied to the fuel electrode as a high- or low-concentrated aqueous solution or as gaseous pure methanol, and methanol is oxidized to carbon dioxide in a catalyst layer of the fuel electrode. Protons produced at this time move to the oxygen electrode through an electrolyte film separating the fuel electrode and the oxygen electrode, and then react with oxygen in the oxygen electrode, thereby to generate water. Reactions occurring in the fuel electrode, the oxygen electrode and the whole DMFC are represented by Chemical Formula 1.
(Chemical Formula 1)
Fuel electrode: CH3OH+H2O→CO2+6e−+6H+
Oxygen electrode: (3/2)O2+6e−+6H+→3H2O
The whole DMFC: CH3OH+(3/2)O2→CO2+2H2O
The energy density of methanol as the fuel for the DMFC is theoretically 4.8 kW/L, which is 10 or more times larger than the energy density of a typical lithium-ion secondary battery. That is, the fuel cell using methanol as a fuel has a good chance of exceeding the energy density of the lithium-ion secondary battery. Therefore, among various fuel cells, the DMFC is the most likely to be used as an energy source for a mobile device, an electric vehicle or the like.
However, the DMFC has such an issue that even though its theoretical voltage is 1.23 V, its output voltage when actually generating electric power is reduced to approximately 0.6 V or less. A cause of a reduction in output voltage is a voltage drop caused by internal resistance of the DMFC, and the DMFC has internal resistance such as resistance accompanied with reactions occurring in both electrodes, resistance accompanied with movement of a substance, resistance generated when protons moves through the electrolyte film and further, contact resistance. Energy which is allowed to be actually extracted as electrical energy through oxidation of methanol is represented by a product of an output voltage during electric power generation and the quantity of electric power flowing through a circuit, so when the output voltage during electric power generation is reduced, the energy which is allowed to be actually extracted is reduced correspondingly. In addition, when the whole amount of methanol is oxidized in the fuel electrode according to Chemical Formula 1, the quantity of electric power which is allowed to be extracted to the circuit through oxidation of methanol is proportional to the amount of methanol in the DMFC.
Moreover, the DMFC has an issue of methanol crossover. Methanol crossover is a phenomenon in which methanol reaches the oxygen electrode side from the fuel electrode side through the electrolyte film due to two mechanisms of a phenomenon in which methanol diffusively moves by a difference in methanol concentration between the fuel electrode side and the oxygen electrode side and an electroosmotic phenomenon in which hydrated methanol is transported by the movement of water caused by the movement of protons.
When methanol crossover occurs, methanol having passed through the electrolyte film is oxidized in a catalyst layer of the oxygen electrode. Although an oxidation reaction of methanol on the oxygen electrode side is the same as the above-described oxidation reaction on the fuel electrode side, the oxidation reaction on the oxygen electrode side causes a reduction in the output voltage of the DMFC. Moreover, methanol is not used for electric power generation on the fuel electrode side, and is wasted on the oxygen electrode side, so the quantity of electric power which is allowed to be extracted to the circuit is reduced correspondingly. Further, the catalyst layer of the oxygen electrode is not a platinum (Pt)-ruthenium (Ru) alloy catalyst but a platinum (Pt) catalyst, so there is such an inconvenience that carbon monoxide (CO) is easily absorbed onto a surface of the catalyst to cause catalyst poisoning.
Thus, the DMFC has two issues of a reduction in voltage caused by internal resistance and methanol crossover and fuel waste caused by methanol crossover, and these issues cause a decline in electric power generation efficiency of the DMFC. Therefore, to improve the electric power generation efficiency of the DMFC, research and development aimed at improving characteristics of materials forming the DMFC, and research and development aimed at optimizing operating conditions of the DMFC have been intensely conducted.
The research aimed at improving the characteristics of the materials forming the DMFC includes research related to the electrolyte film and a catalyst on the fuel electrode side. As the electrolyte film, a polyperfluoroalkyl sulfonic acid-based resin film (“Nafion (a registered trademark)” manufactured by E. I. du Pont de Nemours and Company) is typically used; however, as an electrolyte film having higher proton conductivity and a higher ability to prevent methanol from passing through than the polyperfluoroalkyl sulfonic acid-based resin film, a fluoropolymer film, a hydrocarbon-based polymer electrolyte film or a hydrogel-based electrolyte film and the like have been studied. As the catalyst on the fuel electrode side, research and development of a catalyst having higher activity than a platinum (Pt)-ruthenium (Ru) alloy catalyst which is typically used at present have been conducted.
Such an improvement in the characteristics of the constituent materials of the fuel cell is appropriate as a means of improving the electric power generation efficiency of the fuel cell. However, at present, an optimum catalyst to solve the above-described two issues has not yet been found, and an optimum electrolyte film has not yet been found.
On the other hand, Japanese Unexamined Patent Application Publication No. S59-191265 discloses a fuel cell using a liquid electrolyte (an electrolytic solution) and not needing the electrolyte film. The electrolytic solution may remain stationary between the oxygen electrode and the fuel electrode, or the electrolytic solution may circulate by flowing through a channel arranged between the oxygen electrode and the fuel electrode to outside, and then going back to the channel.
However, when a fuel cell stack system in which a plurality of fuel cell elements are stacked in a vertical direction or a horizontal direction is considered as a fuel cell using an electrolytic solution, the fuel cell has issues that it is more difficult to manufacture the fuel cell than a fuel cell including a solid electrolyte film in related art, and it is difficult to stably generate electric power. It is because unlike the fuel cell using the solid electrolyte film in related art, it is necessary to supply two kinds of fluids, that is, a fuel and an electrolytic solution as a liquid electrolyte to the fuel cell using the electrolytic solution, and, further in the case where the fuel cell stack system is configured, unless two kinds of fluids are supplied substantially uniformly to each of the fuel cell elements, electric power is not stably generated.
Typically, in the case where a fluid flows through the fuel cell stack system, the fluid is sent to a main channel connected to all fuel cell elements so as to be supplied to each of the fuel cell elements through the channel. That is, the fluid is supplied by parallel connection. However, it is extremely difficult to supply the fluid uniformly to each of the fuel cell elements.
First, it is difficult to make the widths and heights of channels of the fuel cell elements 100% uniform. In addition to this, carbon dioxide or the like generated during electric power generation is released into the fluid as bubbles, thereby to disturb the flow of the fluid, so a pressure loss in each of the fuel cell elements is changed due to various factors, thereby a fuel cell in which the fluid easily flows and a fuel cell in which the fluid flows with difficulty are inevitably produced. Means to prevent such a situation and create an environment that the fluid easily flows include allowing a sufficient height of the channel, and the like, but needless to say, this means causes an increase in the thickness of the fuel cell stack system, thereby resulting in an increase in size.
Also, Japanese Unexamined Patent Application Publication No. 2006-164872 discloses that a fluid is supplied to each fuel cell element through an individual pump and a valve. However, in such a configuration in related art, when a fuel cell stack system including 30 fuel cell elements is assumed and two pumps are necessary per fuel cell, 60 pumps are necessary in total. Therefore, devices for supply such as pumps occupy a majority of the fuel cell stack system to cause an increase in size of the fuel cell stack system, so the configuration is extremely unrealistic.
Moreover, there is a common issue in a fuel cell needing an electrolyte film and a fuel cell not needing an electrolyte film in related art. For example, it is necessary to uniformly supply a fuel, an electrolytic solution, or oxygen, air or the like into the fuel cell, and when a flow rate distribution, a pressure distribution or a concentration distribution is locally generated in the fuel cell, the characteristics of the fuel cell is extremely unstable. Therefore, it is essential to design an optimum shape of a channel (microchannel) so as to uniformly supply a liquid or a gas supplied in the fuel cell as a whole.
As the shape of the channel of the fuel cell, there are a large number of kinds such as a serpentine (meandering) shape type as one kind of a serial channel in which an inlet 352A and an outlet 353A are connected by one channel 356A as illustrated in FIG. 41 and a grid type in which a grid-like channel 356B is arranged in a matrix form between the inlet 352A and the outlet 353A as illustrated in FIG. 42. However, the serpentine type channel shape has a major issue. It is because when a gas fuel such as hydrogen or oxygen flows through the channel, the pressure loss is small, but when a liquid fuel such as a methanol aqueous solution or an electrolytic solution such as a sulfuric acid flows through the channel, the pressure loss is pronouncedly increased to cause an increase in the power of a pump for flowing the fluid.
Moreover, in the serpentine type channel, the case where the concentration distribution of a reactive gas or a liquid tends to be generated in the channel and electric power is generated locally under a high utilization rate condition and a low utilization rate condition often occurs, thereby the case tends to become a cause of reductions in the performance and longevity of the fuel cell. That is, the concentration distribution or the like causes catalyst deterioration, thereby to reduce its performance not temporarily but permanently.
As a method of avoiding an increase in pressure loss and deterioration in performance, the introduction of a parallel channel is considered. In the parallel channel, as illustrated in FIG. 43, first, the fluid flows from the inlet 352A to a first main channel 352 to be supplied to a plurality of parallel channels 354 connected to the first main channel 352 at a right angle, and then the fluid joins into a second main channel 353 connected to the exit 353A. In such a parallel channel configuration, compared to the serpentine type as one kind of serial channel, the pressure loss is allowed to be significantly reduced.
However, in the channel configuration, it is extremely difficult to supply the fluid uniformly to the plurality of parallel channels 354. In particular, as illustrated in FIG. 43, in the case where the first main channel 352 and the second main channel 353 have the same widths or depths (heights) as those of the parallel channels 354, the fluid does not flow uniformly through the parallel channels 354.
To allow the fluid to flow through the parallel channels 354 as uniformly as possible, it is necessary to reduce the resistance of the first main channel 352 connected to the inlet 352A and the resistance of the second main channel 353 connected to the outlet 353A to the flow of the fluid to a minimum, and to form a channel configuration where the fluid flows more easily than the parallel channels 354. However, to do so, it is necessary to allow a sufficient height of the channel, so the thickness of a plate forming the channel is inevitably increased to 1 mm or over. As a result, the thickness of the whole fuel cell is increased, and the size of a stack configuration in which the fuel cells are stacked is also increased.
Further, in the fuel cell using the electrolytic solution, the fuel electrode and the oxygen electrode is constantly in contact with the fluid, so there is an issue that deterioration in the electrodes such as a crack or a hole is inevitable. The deterioration in the fuel electrode promotes fuel crossover, and the deterioration in the oxygen electrode causes leakage of the electrolytic solution. Such deterioration in the electrodes is fatal, and the characteristics of the fuel cell are pronouncedly deteriorated.
By the way, in related art, a fine channel formed on a glass substrate or a base such as a plastic film is used as an analyzer, a chemical reaction chip or a biochemistry chip or the like. As a method of connecting a fluid connector (tube) to such a channel, for example, as illustrated in FIG. 44, an extendable member 422 is arranged on an inlet-outlet 431 of a channel 420, and a gap 440 between the inlet-outlet 421 and a fluid connector 430 is filled with the extendable member 422. Moreover, Japanese Unexamined Patent Application Publication No. 2004-58214 discloses that a connection section having adhesion which is fixable to another device is arranged around an opening of a channel.
However, in these configurations in related art, a tube is connected in a direction perpendicular to a surface of the base, so a space in a vertical direction of the base is occupied by connection of the fluid connector, thereby the bases to which the fluid connectors are connected are not allowed to be stacked in parallel.
There is a method in which the thickness of the base and the thickness of the tube diameter of the fluid connector connected to the base are substantially equal to each other, and the fluid connector is connected to a side surface of the base with an adhesive or the like; however, in addition to needing care for adhesion, the fluid connector is not allowed to be connected to a substrate with a smaller thickness than the tube diameter (outside diameter) of the fluid connector.